The fruit fly Drosophila melanogaster has 18 nuclear receptor (NR) genes, significantly fewer than the 48 genes found in humans, spanning all vertebrate NR subclasses and encoding orthologs of key human receptors, including RXR (USP in flies), NGFI-B/NURR (DHR38), ERR (dERR), SXR (DHR96), and HNF4 (dHNF4). These characteristics of Drosophila NRs, combined with an extensive collection of genetic and genomic tools, establish the fly as an ideal model system for studying the molecular mechanisms of NR regulation and function. In this proposal, we focus on 2 major NR-regulated biological pathways: steroid-triggered maturation and lipid metabolism. Like vertebrates, Drosophila maturation is triggered by steroid hormones and their receptors, via complex transcriptional cascades that were identified and characterized in the fly. In contrast, no studies to date have addressed roles for Drosophila NRs in lipid metabolism. We will study the steroid regulation of a poorly understood transition that occurs during the last larval stage, when the animal commits to terminating its juvenile growth phase and initiating maturation via metamorphosis. We will identify genes regulated by alpha-ecdysone (E, the primary secreted steroid in Drosophila and the precursor to the active hormone 20-hydroxyecdysone, or 20E) and ask if these effects go through the DHR38 E receptor and its RXR partner, USP. We will determine if DHR38 and EcR function as partially redundant steroid receptors to initiate maturation. We will determine if dERR is transcriptionally controlled by 20E, examine its possible roles in initiating metamorphosis, and determine whether it controls lipid or sterol metabolism. We will follow up on the observation that DHR96 co-purifies with cholesterol and that DHR96 null mutants require cholesterol for their survival on minimal medium. Finally, we will characterize the expression and function of dHNF4 to define its roles in lipid metabolism and development. Metabolic profiling and microarrays will be used to gain a more complete understanding of the molecular mechanisms of dERR, DHR96, and dHNF4 function. Selected genes will be defined as direct regulatory targets. These studies will expand our understanding of NR signaling pathways with direct implications for how the orthologous NRs function in humans, as well as their contributions to critical human diseases associated with NR dysfunction, including cardiovascular disease, diabetes, and obesity.